An oxygen sensor, such as a universal exhaust gas oxygen (UEGO) sensor, may be positioned in an exhaust system of a vehicle to detect an air-fuel ratio (AFR) of exhaust gas from an internal combustion engine of the vehicle. The oxygen sensor readings may be used to adjust operation of the internal combustion engine, for example, by altering an amount of fuel injected in order to reach a desired AFR. Therefore, degradation of the oxygen sensor may result in degraded fuel injection control, which may lead to increased emissions, reduced vehicle drivability, and reduced fuel economy.
Oxygen sensors are often installed using a sealant, for example, a silicone sealant. However, exhaust gas may reach a temperature that is hot enough to cause the sealant to release gases—a phenomenon known as “off-gassing.” Sealant off-gassing may degrade the oxygen sensor by interfering with oxygen concentration measurements. In contrast to generic degradation, for example, due to sensor aging, degradation due to sealant off-gassing may be rapid.
Other attempts to address oxygen sensor degradation due to sealant off-gassing include applying a bias voltage to adjust a measurement output of the oxygen sensor. One example approach is shown by Zarkhin et al. in U.S. Pat. No. 6,382,013 B1. Therein, degradation due to sealant off-gassing is detected by an inversion of the measurement output (e.g., from positive volts to negative volts), and the measurement output is adjusted using a predetermined bias voltage applied to a sensor return. The inventors therein note that the absolute value of the sensor measurement remains accurate despite the inversion.
However, the inventors herein have recognized potential issues with such methods. As one example, sealant off-gassing may cause the oxygen sensor to read rich (e.g., due to the released gases diluting the concentration of oxygen in the exhaust gas) or lean (e.g., due to sealant gases coating the sensor). Therefore, the effects of sealant off-gassing may be more complex than causing an inversion in oxygen sensor output voltage.
In one example, the issues described above may be addressed by a method comprising, responsive to a change in a demand for fueling an engine without a change in demand for engine output at an engine exhaust gas temperature greater than a threshold temperature, indicating degradation of an exhaust gas oxygen sensor, connected to an exhaust system of the engine with a sealant, due to sealant off-gassing; and correcting measurements of the exhaust gas oxygen sensor in response to the indication. In this way, oxygen sensor degradation due to sealant off-gassing may be identified, and the oxygen sensor measurements may be compensated.
As one example, indicating degradation of the exhaust gas oxygen sensor due to sealant off-gassing further comprises indicating exhaust gas oxygen sensor degradation due to sealant off-gassing without sensor coating when the exhaust gas oxygen sensor measures a rich air-fuel ratio of the engine exhaust gas coupled with a decreased fueling demand and indicating exhaust gas oxygen sensor degradation due to sealant off-gassing with sensor coating when the exhaust gas oxygen sensor measures a lean air-fuel ratio of the engine exhaust gas coupled with an increased fueling demand. In this way, degradation due to off-gassing without sensor coating may be distinguished from degradation due to the exhaust gas oxygen sensor becoming coated with sealant gasses. Although oxygen measurements made by an exhaust gas oxygen sensor degraded due to sealant off-gassing without sensor coating may be corrected in the same way as oxygen measurements made by an exhaust gas oxygen sensor degraded due to sealant off-gassing with sensor coating, exhaust gas oxygen sensor degradation due to sealant off-gassing with sensor coating may be irreversible. Therefore, it is advantageous to distinguish between the two forms of degradation due to sealant off-gassing (e.g., with and without sensor coating) to identify when sensor replacement is indicated.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.